The technique of modifying thermoplastic resins with rubbers to improve the impact strength of the resins has been established. Examples of such modified resins include ABS resins obtained by reinforcing styrene-acrylonitrile resins (AS resins) with a butadiene rubber and AAS resins obtained by reinforcing AS resins with an acrylic rubber.
A silicone rubber may be usable as such a base rubber ingredient. However, a mere blend of a polyorganosiloxane with a thermoplastic resin has insufficient impact resistance because of poor compatibility between these ingredients. Although the technique of grafting a vinyl monomer onto a rubber as in the production of ABS resins is needed to obtain good compatibility, polyorganosiloxanes generally have low reactivity with vinyl monomers and hence formation of a graft copolymer is difficult. Several techniques have been disclosed for forming this kind of graft copolymer.
For example, a technique of polymerizing a vinyl monomer in the presence of a polyorganosiloxane containing a vinyl or allyl group to form a graft copolymer having improved impact strength is proposed in JP-A-50-109282. (The term "JP-A" as used herein means an "unexamined published Japanese patent application.")
In JP-A-52-130885 is proposed a technique of improving the impact strength of a vinyl polymer using a polyorganosiloxane containing a mercapto group in place of a polyorganosiloxane containing a vinyl or allyl group.
In JP-A-60-252613, JP-A-61-106614, and JP-A-61-136510 is proposed a technique of polymerizing a vinyl monomer in an emulsion of a polyorganosiloxane containing an acrylic or methacrylic group to thereby obtain a graft copolymer having a high graft efficiency and excellent impact strength.
Further, a technique of graft-polymerizing a vinyl monomer onto a polyorganosiloxane containing a vinylphenyl group to obtain a graft copolymer excellent in impact strength and slidability is proposed in JP-A-2-8209.
With respect to crosslinking agents, a technique of improving the impact resistance of a graft copolymer by using a polyfunctional silane crosslinking agent, e.g., methyltrimethoxysilane or tetraethoxysilane, as an additional crosslinking agent is proposed in JP-A-60-252613 and JP-A-2-8209.
However, in the case of using a polyorganosiloxane containing any of the above-described graft crosslinking agents and not containing any other crosslinking agent, the resin obtained has insufficient fatigue resistance because the modified polyorganosiloxane has a large intermolecular space and hence a weak intermolecular force. In addition, the range of applications of the thus-obtained resin is limited because the polyorganosiloxane swells in organic solvents such as n-hexane and toluene and fuels such as gasoline and illuminating kerosene.
The use of a polyfunctional silane crosslinking agent such as the above-described compounds as an additional crosslinking agent also has a problem that the pliability of polyorganosiloxanes is impaired due to the reduced distance between crosslinking sites and, hence, slidability and fatigue resistance are reduced.